Method of making an improved graded collector for inductive loads

ABSTRACT

A bipolar transistor having an improved collector structure includes a buried region of the same conductivity type as the collector region spaced from the base region and having a laterally graded impurity concentration with the lowest below the center of the emitter .region. An integrated circuit may include transistors having the buried collector region of the diminishing lateral impurity concentration below the center of its emitter as well as having transistors with a uniform lateral impurity concentration below the total lateral extent of the emitter. A method of achieving the unique collector region includes forming at least a first collector region of a first conductivity type as two lateral portions of substantially uniform lateral impurity concentration with a space therebetween in a substrate of a second conductivity type and heating to form the buried collector structure of diminishing lateral impurity concentration. This is followed by forming a collector layer on the substrate, forming a base region in the collector region and an emitter region in the base region over the space in the buried collector region. If the integrated circuit includes transistors which do not have the unique graded collector, the buried collector region for these transistors is formed by introducing impurities having substantially uniform lateral impurity concentration in the substrate.

This is a division of application Ser. No. 07/674,147 filed on Mar. 25,1991 now U.S. Pat. No. 5,311,054.

BACKGROUND AND SUMMARY OF THE INVENTION

The present invention relates generally to bipolar transistors and morespecifically to an improved transistor for use with inductive loads.

A bipolar transistor formed according to the prior art is illustrated inFIG. 1 as including a P substrate 10 having an N buried subcollector 12formed therein between the substrate 10 and an N- epitaxial collectorregion 14. The base-region 16 is a P type with extrinsic base portion17. An N+ emitter 18 is formed in the base region 16. The impurityconcentration distribution of the epitaxial collector 14 and thesubcollector 12 is shown specifically in FIG. 2 by curve A. The abrupttransition between the collector 14 and the buried collector region 12minimizes the amount of the collector region 14 of the low impurityconcentration between the base region 16 and the buried collector 12.Thus the series resistance of the collector is very low. The abrupttransition results from the processing with a minimum of out-diffusionof the buried collector 12 into the epitaxial layer collector 14. Thisalso reduces the variability in the active portion of the collector 14between the buried layer 12 and the base region 16.

During turn-off, due to the voltage drop from the lateral Flow of basecurrent, the edge of the base region adjacent 17 turns off faster thanthe portion of the base region 16 below the emitter region 18. This isillustrated by the variable resistance and the arrows in FIG. 1. Lowcollector resistance allows a strong focus of current at the center ofthe emitter during inductor turn-off. Thus there is a hot spot under thecenter of the emitter region. The abrupt profile of the N-/N+ collectorregions 14/12 creates higher fields at higher emitter currents, Asecondary breakdown results from inductive loads when the transistor isturned off. The lack of defocusing during the turn-off is verydetrimental because of the high fields and currents below the emitter,causes transistor burn-out at hot spots.

One approach of the prior art to reduce the peak current density underthe emitter is to spread fields by providing a graded collector region14. This is produced by a substantial out-diffusion of the subcollector12 into the epitaxial layer 14 after it is formed. To achieve the sameactive collector region 14 between curves A and B of FIG. 2, the sub orburied collector region 12' is substantially greater in depth than thatof buried collector region 12 of curve A, i.e., more epitaxialdeposition is required. Also because of the amount of time that must bediffused as well as its depth, the controlability of the resultingregion 14 is substantially less for graph B as it is for graph A. Also,the effective resistivity of the combination of collector portion 14 and12' is substantially greater than that of 14 and 12, thereby increasingthe overall collector resistance, which also limits the performance andpacking density elsewhere on an integrated circuit.

Thus, there is a need for providing the turn-off ruggedness of a gradedcollector region with the low resistance of an abrupt collector region.Thus, it is an object of the present invention to provide a collectorregion which combines the advantages of a graded collector region and anabrupt collector region.

Another object of the present invention is to provide an improvedbipolar transistor having a collector region designed specifically forinductive loads.

Still another object of the present invention is to provide a transistorstructure which can be individually tailored in an integrated circuitwithout extra steps.

These and other objects are achieved by providing a buried region of thesame conductivity type as the collector region spaced from the baseregion and having a substantially uniform lateral impurity concentrationexcept below the center of the emitter region where it has a decreasingimpurity concentration range, This defocuses and spreads the field andcurrent concentration in the collector directly below the emitter andyet maintains a low collector resistance, The impurity concentration ofthe buried collector region below the emitter region decreases laterallytowards below the center of the emitter region. The impurityconcentration of the buried region below the center of the emitterregion is less than the remainder of the buried collector region and maybe slightly higher or lower than the collector region or may be theimpurity concentration of the collector region. Where the substrate is adifferent impurity conductivity type than the collector region, theburied collector region extends into the collector region and thesubstrate. An integrated circuit may include transistors having theburied collector region of the diminishing lateral impurityconcentration below the center of its emitter region as well as havingtransistors with a uniform lateral impurity concentration below thetotal lateral extent of its emitter region.

A method of achieving the unique collector region includes forming atleast a first collector region of a first conductivity type as twolateral portions substantially uniform lateral impurity concentrationwith a space therebetween in a substrate of a second conductivity type.This is followed by forming a collector layer of a first conductivitytype on the substrate, forming a base region of the second conductivitytype in the collector region and forming an emitter region of the firstconductivity type in the base region over the space in the buriedcollector region. The lateral portions of the buried collector regionare formed having an impurity concentration below the emitter decreasinglaterally towards the center of the emitter. The impurity concentrationat the center is less than the remainder of the buried collector portionand maybe either slightly higher than the collector layer or be theimpurity concentration of the collector layer.

The buried collector region is formed by introducing impurities of afirst conductivity type into the substrate in two regions with a firstspace therebetween. It is followed by heating the substrate to diffusethe impurities vertically and laterally into the substrate to form thetwo portions of substantially uniform lateral impurity concentrationexcept in the first space which has diminishing lateral impurityconcentrations, thus providing a laterally graded collector.Alternatively, impurities of the first buried collector region may beformed by introducing impurities of the first conductivity type andfirst concentration into the substrate in two regions with a first spacetherebetween. Impurities of a first conductivity type and a secondconcentration lower than the first concentration or a faster diffusionrate are introduced into the first space. Subsequently, the substrate isheated to diffuse impurities into the substrate to form the two portionsof the same substantially uniform lateral impurity concentrationseparated by the first space of lower substantially uniform lateralimpurity concentration. If the integrated circuit is to includetransistors which do not have the unique graded collector, the buriedcollector region for these transistors is formed by introducingimpurities having substantially uniform lateral impurity concentrationin the substrate and forming with the base and emitter region thereoversuch that the buried collector region has substantially uniform impurityconcentration under the emitter region.

Other objects, advantages and novel features of the present inventionwill become apparent from the following detailed description of theinvention when considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-section of a bipolar transistor of the prior art;

FIG. 2 is a graph representing the impurity concentration versus depthcomparing the prior art to the present invention;

FIG. 3 is a cross-sectional view of an integrated circuit including atransistor of the prior art and the present invention;

FIGS. 4-6 are cross-sectional views of a wafer during various stages offabrication according to the principles of the present invention;

FIGS. 7 and 8 are cross-sectional views of a modified process accordingto the principles of the present invention;

FIG. 9 is a diagram of the current flow during turn-off of a transistorof the present invention;

FIG. 10 is a graph comparing the current density versus lateral distancebetween the transistor of the present invention and the prior art; and

FIG. 11 is a graph of the electric field versus depth under the emittercomparing the present invention with that of the prior art,

DETAILED DESCRIPTION OF THE DRAWINGS

An integrated circuit is illustrated in FIG. 3 as including a prior artbipolar transistor 20 and a bipolar transistor 20" according to theprinciples of the present invention. As in FIG. 1, the prior arttransistor 20 includes a substrate 10 with a buried collector region 12,collector layer or region 14, a base region 16, 17 and an emitter region18. The bipolar transistor 20" is also built in the substrate 10 havinga buried collector region 12", a collector layer or region 14, a baseregion 16, 17 and an emitter region 18. For purposes of illustration,both transistors are shown as NPN transistors wherein the substrate 10is P, the buried region 12 and 12" are N+, the collector layer 14 is N-,the base regions 16, 17 are P and the emitter 18 is N+. Those regionshaving common characteristics in transistor 20 and 20" have the samereference number. The lateral impurity concentration of buried collectorregion 12 of transistor 20 is substantially uniform whereas the lateralimpurity concentration of buried collector region 12" is substantiallyuniform except under the emitter region 18 where it decreases to thevalue of N- of the collector 14 or some value slightly higher than theN- value of collector 14 but lower than the N+ value of the remainder ofthe buried collector 12". Alternatively, the buried collector region mayalso have an impurity concentration lower than N- of the collector 14,although less desirable in some applications.

As illustrated in FIG. 2, the abrupt change of transistor 20 isillustrated by the curve A which shows impurity concentration of thecollector at depths from the surface of the wafer. Curve 12' is thecollector concentration where the buried layer 12 has been substantiallyout-diffused into the collector layer 14. This is not illustrated in thefigures other than the graph of FIG. 2. The desired concentration forthe collector region 12" is illustrated by the cross-hatched regions anddepends upon where the cross-section is taken. The collectorconcentration below the base portion 17 and not under the emitter region18 is illustrated by the prior art curve A. A cross-section takenthrough the center of the emitter 18 through the collector regions 14and 12" is illustrated by curve C. The cross-sections between these twoextremes are the cross-hatched area. In curve C, the minimum impurityconcentration of the buried layer 12" rises slightly above that of thecollector region 14 and then decreases rapidly as does curve A. This isin contrast to a substantially uniform rate of grading of curve B.Although the buried layer portion 12" is illustrated in FIG. 3 as notextending totally below the emitter 18, because of the lateral diffusioncharacteristics of the impurities, there is generally a slight increasein impurity concentration of layer 14 even at the center of the emitterregion 18, although not always required.

The process for producing the integrated circuit of FIG. 3 begins as inFIG. 4 by providing impurities of the N conductivity type in the Psubstrate 10. The portions where high performance transistors are to beformed, the impurity is introduced uniformly. In the regions where theimproved collector region of transistor 20" is to be formed, theimpurity is introduced with a space therebetween above which the emitterregion is to be formed. This is illustrated as regions 12 and 12" inFIG. 4. The substrate is then subjected to a heating step to diffuse theimpurities to form the regions as illustrated in FIG. 5. The series oflines illustrate the impurity distribution. As noted, the region 12 fortransistor 20 has a substantially uniform lateral distribution whereasthe region 12" has two portions of substantially uniform lateralimpurity concentration and diminishes toward the center.

Next, the collector layer 14 is formed preferably by epitaxialdeposition. This results in some out-diffusion of the buried regions 12and 12" into the epitaxial layer 14. The impurity concentration isillustrated in FIG. 6 to represent the final distribution after all theprocess even though they are not shown-specifically in FIG. 6. Byperforming the heat step of FIG. 5 prior to forming the epitaxial layer14 of the collector, only a small portion of the epitaxial layer 14includes out-diffusing impurities from the buried region 12 and 12".Thus, the control factor of the prior art transistor of FIG. 1 or 20 isincluded in the 20" transistor. This is distinguished from the curve Band 12' of the prior art uniformly graded collector region which is verydifficult to control.

To even further control the impurity concentration of the buriedcollector region 12" to match the curve C of FIG. 2, a modification isshown in FIG. 7. After forming the buried N+ regions 12 and 12" with aspace therein, a second N type region 13 in the opening between 12" isformed. The impurity concentration of the region 13 is lower than thatof 12" but generally higher than that of the resulting N- impurityconcentration of the epitaxial region 14, although the impurityconcentration may be equal to or lower than the impurity concentrationof epitaxial region 14. A heating step is performed and an epitaxiallayer 14 is formed thereon resulting in the structure of FIG. 8.Transistor 20 then has a buried collector region 12 of uniform lateralimpurity concentration whereas transistor 20" has a buried region 12" ofa first uniform lateral impurity concentration except at its center 13which has a second uniform lateral impurity concentration of a lowerimpurity concentration than 12".

Although the thickness of region 13 is shown thinner than region 12" inFIG. 8, the process could be performed to produce equal depths and stillprovide lateral grading. In the process of FIG. 7, the impurities forregion 13 could have a faster diffusion rate than the impurities ofregion 12". For example, region 12" could be arsenic and region 13phosphorus. The deposition or implantation concentration may be equaland result in unequal diffused impurity concentration. Also, the region13 can be deposited or implanted and diffused before the processing ofregion 12", thereby resulting in equal or greater depth for region 13than region 12" and lower impurity concentration.

The current flow of transistor 20" according to the present inventionduring turn-off is illustrated in FIG. 9. There is no excessiveconcentration of the current or forming of hot spots below the center ofthe emitter. Also, the edge of the transistor turns off first asdiscussed with respect to FIG. 1.

The current density under the emitter of transistors 20 and 20" isillustrated in FIG. 10. The distance is the distance along the surfacewith the center line of the transistor being illustrated. The family ofcurves are for the depths of 0.3 microns, 0.5 microns and 0.7 microns.The prior art or abrupt collector transistor 20 is represented by thedashed line graphs whereas the transistor of the present invention 20"is illustrated by the solid line graphs for each of the family ofcurves.

The comparison of the electric field under the emitter of transistor 20and 20" are illustrated in the graphs of FIG. 11. The solid linerepresents transistor 20" while the dashed graphs illustrated transistor20. The roman numerals I, II and III illustrate the cross-sections takenin FIG. 3. I and I" are taken at the center of the emitter region andillustrate a substantial reduction in the electric field below thecenter of the emitter region. This aids the transistor in operating withthe large currents resulting from the turn-off of the transistor whenoperating with an inductive load. Comparing the other curves of II toII" and III to III", it is noted that the electric fields of 20" areslightly higher than that of the transistor 20 of the prior art. Thisresults from the reduction of the electric field and reduced currentfocus at the center of the emitter. These small increases in theelectric field at positions II and III do not substantially change theperformance of the transistor.

Although the present invention has been described and illustrated indetail, it is to be clearly understood that the same is by way ofillustration and example only, and is not to be taken by way oflimitation. The spirit and scope of the present invention are to belimited only by the terms of the appended claims.

What is claimed is:
 1. A method of fabricating an integrated circuitcomprising:forming a first buried collector region of a firstconductivity type having two lateral portions of substantially uniformlateral impurity concentration with a space there between in a substrateof a second conductivity type; forming in said substrate and over saidfirst buried collector region, a first collector region of said firstconductivity type and of a concentration; forming over said firstcollector region a first base region of said second conductivity type;forming in said first base region and over said space between the twolateral portions of the first buried collector region a first emitterregion of said first conductivity type; and forming in said spacebetween said two lateral portions of said first buried collector region,a buried graded collector region, said buried graded collector regionhaving a central portion spaced from the two lateral regions, saidburied graded collector region having a graded concentration thatdecreases laterally in a direction toward said central portion and saidcentral portion having an impurity concentration approximately equal tothe concentration of said first collector region.
 2. A method accordingto claim 1 wherein forming said first buried collector regionincludes:introducing impurities of said first conductivity type in saidsubstrate in said two lateral portions with said space therebetween; andheating said substrate to diffuse said impurities vertically andlaterally into said substrate to form said two lateral portions ofsubstantially uniform lateral impurity concentration except in saidspace which has a graded lateral impurity concentration.
 3. A methodaccording to claim 1 wherein forming said first buried collector regionincludes:introducing impurities of said first conductivity type having afirst impurity concentration in said substrate in said two lateralportions with said space therebetween; introducing impurities of saidfirst conductivity type and a second impurity concentration lower thansaid first impurity concentration in said substrate in said space;heating said substrate to diffuse said impurities into said substrate toform said two lateral portions of said substantially uniform lateralimpurity concentration separated by said space of a lateral uniformimpurity concentration less than the concentration of said two lateralportions.
 4. A method according to claim 1 wherein forming said firstburied collector region includes:introducing first impurities of saidfirst conductivity type having a first diffusion rate in said substratein said two lateral portions with said space therebetween; introducingsecond impurities of said first conductivity type having a seconddiffusion rate lower than said first diffusion rate in said substrate insaid first space; and heating said substrate to diffuse said first andsecond impurities into said substrate to form said two lateral portionsof the said substantially uniform lateral impurity concentrationseparated by said space of a lateral uniform impurity concentration lessthan the concentration of said two lateral portions.
 5. A methodaccording to claim 1 further comprising:forming in said substrate asecond buried collector region of a first conductivity type havingsubstantially uniform lateral impurity concentration; forming a secondbase region of said second conductivity type in said first collectorregion above said second buried collector region; and forming a secondemitter region of said first conductivity in said second base regionabove said second buried collector region.